An accurate converter model is the foundation of AC/DC system fault analysis and harmonic calculation.Based on the dynamic switching characteristics of inverter and fault response characteristics of DC control system of various fault-types，a DC system model able to directly interface with AC systems is developed for AC/DC system fault analysis and harmonic calculation.By referring to the AC and DC power grid harmonic equivalent network，an high voltage direct current（HVDC）system harmonic calculation method suitable for AC asymmetric fault condition is proposed.The method is used in the harmonic calculation of CIGRE HVDC system and Guiguang Ⅱ HVDC system.Compared with simulation results using PSCAD/EMTDC software，the results obtained show that the proposed model and method are accurate and effective，providing a basis for quantitative analysis of harmonic mitigation，filter configuration and protection setting of the AC/DC system.